This invention relates to moulds and has particular reference to open ended continuous casting moulds.
Continuous casting moulds basically comprise an open ended box into which molten metal is poured at one end and from which solid rod, slab or tube is extracted at the other end, the metal solidifying within the mould on a continuous basis. The system incorporating the mould is so arranged that the withdrawal speed of solidified metal at the bottom of the mould exactly equals the inflow of molten metal at the top so that the mould is in a steady state. Continuous casting moulds are, of course, totally different from ordinary moulds in which metal is poured into the mould to fill it and solidification takes place within the mould, the solidified metal then being removed from the mould in one piece.
Although in theory a very simple concept, continuous casting has proved to be quite difficult to utilise in commercial practice. There is a considerable amount of technology needed to manufacture and use continuous casting moulds on a commercial basis. There are a number of moulds in commercial use and there are very many more moulds which have been proposed although never used in practice.
In British Pat. No. 822,578 there is disclosed a continuous casting slab mould which utilises thin graphite sheets connected at their top and bottom edges only to a thin copper backing sheet. The specification states that because of the temperature differences across both the graphite and the copper the graphite and copper will distort in such a manner as to form a very good thermal contact between the graphite and copper. Unfortunately, however, graphite creeps at high temperature such that the stress between the graphite and copper can relax in use, permitting the graphite to move away from the copper sufficiently to increase dramatically the thermal resistance of the graphite copper interface. One of the significant problems about graphite lined moulds relates to the air gap which normally exists between the copper and graphite. At a temperature of 500.degree. C. air has a thermal resistance 7500 times that of copper and this means that, for example, an air gap of 0.001 in would correspond to a thickness of 71/2 in of copper.
There are a number of continuous casting moulds used in practice in copper refining but although they have their advantages they also suffer from a number of disadvantages. In one known mould, which is a solid copper block having water cooling channels bored in it, a mould cavity being chromium plated and all of the primary water impinging on the withdrawing slab, has an advantage in that it is robust. Unfortunately it requires continuous lubrication to prevent adhesion between the cavity wall and the product. The lubricant causes product surface imperfections reducing yield.
Lubrication addition rates significantly less than the norm greatly exacerbates the adhesion problems resulting in a safety hazard.
The mould described in British Pat. Nos. 853,853 and 853,854 is a complex integral water-cooled graphite block.
Another type of mould is basically a copper inner box with an outer steel water jacket, there being 4 graphite plates forming a lining in the mould cavity.
A further mould utilises 4 graphite blocks bound together to form the mould cavity, with either a wide copper sheet pulled tight with a turn buckle or a copper tape tightly helically wound round the outside of the composite. Water is sprayed from a manifold onto the copper binding and secondary cooling is provided by a separate circuit.
Yet another mould basically comprises an inner copper mould with an outer steel water jacket in two pieces which bolt together to form a water cooling circuit. All the primary cooling water passes through holes at the base of the mould to impinge on the withdrawing slab thus becoming secondary cooling water. This gives advantages in that the design is simple but unfortunately the casting rate is low, the mould is prone to mechanical damage and also prone to distortion.